Heretofore, a mobile communication system as shown in FIG. 6 has been known in which multiple cells #11 to #32 are formed and a mobile station UE is configured to communicate in an appropriate cell.
In the conventional mobile communication system, a radio network controller RNC is configured to determine a cell with which the mobile station UE should establish a radio link, on the basis of information such as reception quality of each cell notified by the mobile station UE, and then to enable the mobile station UE to perform communications by establishing a radio link between the mobile station UE and a radio base station NodeB in this determined cell.
In a W-CDMA type mobile communication system, a scrambling code is assigned to each cell, and the mobile station UE is configured to identify a cell in the downward radio link by searching the scrambling code.
Moreover, as shown in FIGS. 7(a) and 7(b), when traveling, the mobile station UE can continue communication without disconnecting the radio link, by sequentially switching the radio links with the radio base stations NodeB from one to another.
Furthermore, for each cell, the radio network controller RNC previously manages information (called neighboring cell information below) on neighboring cells (i.e., one or more cells which the mobile station UE communicating in the cell may possibly select next).
The radio network controller RNC is configured to sequentially update a list of candidate cells for addition (called addition-candidate cell list below) and to notify the mobile station UE of the update result, every time the one or more cells with which the mobile station UE is establishing a radio link (called communicating cell below) change. The addition-candidate cells are the neighboring cells that the mobile station UE may possibly select next, and the list is updated according to neighboring cell information managed for each communicating cell.
As a result, the communicating mobile station UE does not need to search for the next cell to establish a radio link with from among the entirety of the possible cells existing in the mobile communication system, but only needs to search for the next cell to establish the radio link with from among neighboring cells included in the addition-candidate cell list notified by the radio network controller RNC. In this way, power consumption and time required for the cell search can be reduced.
For example, assume a case where a cell (home cell) covered by a small radio base station NodeB which can be used only by a specific mobile station UE is located within a cell (macro cell) in a communication area covered by a radio base station NodeB which can be used by a general mobile station UE. In this case, although the general mobile station UE, is unable to access the home cell, the radio network controller RNC transmits neighboring cell information including the home cell and the general mobile station UE measures reception quality in the home cell on the basis of the neighboring cell information. Hence, there has been a problem that the mobile station UE wastefully consumes power for such measuring.
Additionally, in the WCDMA type mobile communication system, the maximum number of cells that can be included in the neighboring cell information is 32. Accordingly, when there are many home cells in the macro cell, the entirety of the home cells cannot be included as the neighboring cells of this macro cell. This is a constraint on execution of handover from the macro cell to the home cell, and may cause a problem of poor continuity in a communication service.